The present invention relates to variable displacement hydraulic pumps having a rotating group and a tiltable swashplate for varying the displacement of the rotating group, and more particularly, to a "wide band neutral" arrangement for such pumps, i.e., a device which substantially eliminates flow from the pump, when the swashplate is close to neutral, even if the pump is not at absolute zero displacement.
Although the hydraulic pump for use with the present invention may include various types of rotating groups, it is especially advantageous when used with a rotating group of the "axial piston" type, i.e., one which includes a rotating cylinder barrel defining a plurality of cylinders, and a piston reciprocable within each cylinder. Therefore, the present invention will be described in connection with such an axial piston pump.
Among the types of axial piston pumps known to those skilled in the art is one in which the tiltable swashplate includes a pair of transversely opposed trunnions which are rotatably supported, relative to the pump housing, by suitable bearing means. A pump of the type described is sometimes referred to as a "trunnion pump"
Although the present invention may be used in axial piston pumps of the trunnion type, as illustrated and described in U.S. Pat. No. 5,358,388, assigned to the assignee of the present invention, and incorporated herein by reference, the invention is even better suited for use in pumps of the "swash and cradle" type, and will be described in connection therewith. Swash and cradle axial piston pumps may be better understood by referring to U.S. Pat. No. 5,590,579, also assigned to the assignee of the present invention, and incorporated herein by reference.
Changes in displacement of an axial piston pump (by changing the tilt angle of the swashplate) may be accomplished either by an appropriate servo mechanism or by a manual input. In either case, it is important for the pump to be able to achieve true neutral, such that there is no substantial flow of pressurized fluid out of the pump when the vehicle operator selects neutral operation of the pump. As is well known to those skilled in the art, the inability of a variable displacement axial piston pump to achieve neutral is extremely undesirable, especially in a vehicle propel system, because even a small flow of pressurized fluid may result in vehicle "creep", i.e., unintended movement of the vehicle, which at the very least, can be annoying to the operator, and may in some situations also be potentially dangerous.
Typically, if displacement changes are accomplished by a servo mechanism, the servo mechanism itself may include an appropriate centering device, i.e., a device which biases the pump displacement toward zero, in the absence of some sort of input displacement command. The wide band neutral arrangement of the present invention may be used advantageously with a servo controlled pump because, typically, there are limitations in accuracy of the return-to-neutral mechanism within the servo mechanism.
However, in the case of a pump which has its displacement varied manually, it is generally recognized as being essential to provide some sort of neutral centering mechanism which will insure effective neutral of the swashplate (and absolute zero flow from the pump) whenever the manual input member is at or very near its neutral position.
Various neutral centering devices have been designed by those skilled in the art. Unfortunately, many of the prior art neutral centering devices have been either complicated and expensive, or difficult to assemble, or have provided insufficient biasing force toward neutral, whenever operating near, but not at precisely neutral. For example, the neutral centering devices of the type illustrated and described in U.S. Pat. Nos. 4,584,926 and 5,207,144 would both appear likely to achieve neutral in a satisfactory manner. However, the ability of the designs of the cited patents to achieve neutral is very tolerance-dependent, and requires the addition of a number of parts which must be located within the pumping chamber, surrounding the rotating group, which may be a packaging problem in some pump designs.